A dual cooling device, comprising two refrigerating cycles of a high-temperature side and a low temperature side cycles, has been used to supply cooling fluid cooled to a very low temperature, in the range of minus tens of degrees C. For example, Japanese Laid-Open Patent Application No. 2004-170007 (hereafter Reference 1) discloses a refrigerator system that uses combined ammonia and CO2 refrigerating cycles, where its high-temperature side refrigerating cycle uses ammonia as a refrigerant and its low-temperature side refrigerating cycle uses CO2 as a refrigerant cooled and liquefied by a cascade condenser. A cooling fluid of a very low temperature, lower than the triple point temperature of CO2 (−56° C.), can be produced by cooling the cooling fluid with the CO2 refrigerant, which has a lower temperature than the triple point temperature of CO2, by allowing the CO2 refrigerant in the low temperature refrigerating cycle to expand to the pressure/temperature level of the CO2 triple point or below. The CO2 refrigerant is reduced to a two-phase mixture of solid CO2 and CO2 gas by means of an expansion valve provided downstream of the cascade condenser for cooling the CO2 refrigerant.
Japanese Laid-Open Patent Application No. 2004-308972 (hereafter Reference 2) discloses a CO2 refrigerator comprising compressors for compressing a CO2 refrigerant to a saturation or supercritical pressure, an expansion device for decreasing the pressure of condensed CO2 refrigerant from a condenser to the pressure/temperature level of the CO2 triple point or below so that the CO2 refrigerant is reduced to a two-phase mixture of solid CO2 and CO2 gas, and a sublimation heat exchanger for allowing the solid CO2 to sublimate by receiving heat from cooling fluid from cooling loads and send the sublimated CO2 gas to the compressors. Further, a cascade heat exchanger cools the high-pressure CO2 gas in the condenser with the refrigerant of a high-temperature side refrigerating cycle such as an ammonia refrigerating cycle.
Although References 1 and 2 disclose supplying cooling fluid to cooling loads, a high-temperature heat source cannot be produced simultaneously. Further, as the CO2 refrigerant is expanded to the pressure/temperature level of the CO2 triple point to reduce the CO2 refrigerant to a two-phase mixture of solid CO2 and CO2 gas, and the latent heat of sublimation of the solid CO2 is used to cool the cooling fluid, the refrigerant flow path can clog or the refrigerant flow path can lose pressure, resulting in unstable operation of the refrigerator.
Accordingly, there remains a need for a CO2 cooling and heating apparatus (hereafter sometimes referred to simply as the apparatus) having an improved coefficient of performance with stable operation control, and capable of producing a high-temperature heat source and a low temperature cold heat source simultaneously by effectively taking the advantages of CO2. A CO2 refrigerant is, not only environmentally friendly since its ozone depleting potential of zero, it is also innoxious, inflammable, and inexpensive. By utilizing the advantage of a heat pump cycle using a CO2 refrigerant that it is very efficient in producing a hot-water supply. The present invention addresses this need.